Verapamil, an investigational drug useful in cardiovascular disease, is known to inhibit calcium movement across cell membranes. Platelet aggregation, potentially important in cardiovascular disease, is dependent upon cellular calcium transport. Therefore, patients receiving verapamil might also experience an alteration in platelet aggregability. Over a wide range of drug blood levels, patients receiving verapamil did not show any alterations in platelet response. However, when verapamil was added to platelet in vitro at 20-40x the clinical blood levels, platelet aggregation was inhibited.